## org.jscience.mathematics.analysis.ode Interface SwitchingFunction

`public interface SwitchingFunction`

This interface represents a switching function.

A switching function allows to handle discrete events in integration problems. These events occur for example when the integration process should be stopped as some value is reached (G-stop facility), or when the derivatives has discontinuities. These events are traditionally defined as occurring when a `g` function sign changes.

Since events are only problem-dependent and are triggered by the independant time variable and the state vector, they can occur at virtually any time. The integrators will take care to avoid sign changes inside the steps, they will reduce the step size when such an event is detected in order to put this event exactly at the end of the current step. This guarantees that step interpolation (which always has a one step scope) is relevant even in presence of discontinuities. This is independent from the stepsize control provided by integrators that monitor the local error (this feature is available on all integrators, including fixed step ones).

Field Summary
`static int` `CONTINUE`
Continue indicator.
`static int` `RESET`
Reset indicator.
`static int` `STOP`
Stop indicator.

Method Summary
` int` ```eventOccurred(double t, double[] y)```
Handle an event and choose what to do next.
` double` ```g(double t, double[] y)```
Compute the value of the switching function.
` void` ```resetState(double t, double[] y)```
Reset the state prior to continue the integration.

Field Detail

### STOP

`static final int STOP`
Stop indicator.

This value should be used as the return value of the `eventOccurred` method when the integration should be stopped after the event ending the current step.

Constant Field Values

### RESET

`static final int RESET`
Reset indicator.

This value should be used as the return value of the `eventOccurred` method when the integration should go on after the event ending the current step, with a new state vector (which will be retrieved through the `resetState` method).

Constant Field Values

### CONTINUE

`static final int CONTINUE`
Continue indicator.

This value should be used as the return value of the `eventOccurred` method when the integration should go on after the event ending the current step.

Constant Field Values
Method Detail

### g

```double g(double t,
double[] y)```
Compute the value of the switching function.

Discrete events are generated when the sign of this function changes, the integrator will take care to change the stepsize in such a way these events occur exactly at step boundaries. This function must be continuous, as the integrator will need to find its roots to locate the events.

Parameters:
`t` - current value of the independant time variable
`y` - array containing the current value of the state vector
Returns:
value of the g function

### eventOccurred

```int eventOccurred(double t,
double[] y)```
Handle an event and choose what to do next.

This method is called when the integrator has accepted a step ending exactly on a sign change of the function, just before the step handler itself is called. It allows the user to update his internal data to acknowledge the fact the event has been handled (for example setting a flag to switch the derivatives computation in case of discontinuity), and it allows to direct the integrator to either stop or continue integration, possibly with a reset state.

If `STOP` is returned, the step handler will be called with the `isLast` flag of the `handleStep` method set to true. If `RESET` is returned, the `resetState` method will be called once the step handler has finished its task.

Parameters:
`t` - current value of the independant time variable
`y` - array containing the current value of the state vector
Returns:
indication of what the integrator should do next, this value must be one of `STOP`, `RESET` or `CONTINUE`

### resetState

```void resetState(double t,
double[] y)```
Reset the state prior to continue the integration.

This method is called after the step handler has returned and before the next step is started, but only when `eventOccurred(double, double[])` has itself returned the `RESET` indicator. It allows the user to reset the state vector for the next step, without perturbing the step handler of the finishing step. If the `eventOccurred(double, double[])` never returns the `RESET` indicator, this function will never be called, and it is safe to leave its body empty.

Parameters:
`t` - current value of the independant time variable
`y` - array containing the current value of the state vector the new state should be put in the same array